1. Field of the Invention
The present invention relates to a suck-back pump used to prevent extra drops from falling from the tip of pipe of the like after a constant amount of liquid has been discharge therefrom.
2. Description of Prior Art
Discharge of liquid of a predetermined accurate amount is required in various arts. It is one the important factors to be considered that any drops must be prevented from being discharged, after the liquid has been discharged in the predetermined amount. For example, in photolithography utilized to form a fine pattern on the surface of a silicon wafer in manufacturing an integrated circuit, the wafer surface must be uniformly coated with photoresist. The coating is normally attained by dropping a predetermined amount of photoresist in the form of drops on the wafer and then rotating the wafer to spread the photoresist uniformly thereover under centrifugal force. In such a process, if any excessive amount of photoresist is dropped on the wafer surface, a coated film would be irregular in thickness resulting in unwanted troubles during any subsequent process.
To prevent the above problem, a method has been used in which some suction is created within the liquid delivery passage to draw an excessive photoresist from the delivery nozzle. Such a suction has been accomplished by at first pressing an elastomeric tube such as a Teflon tube in the diametrical direction during feeding of amount of photoresist out of the tube and then releasing the tube to return its initial form to create a suction, or by providing a bellows type pump in the delivery passageway for drawing the excessive liquid into the interior of the tube. In either of these piror art means, however, there is a remarkable degradation. If the elastomeric tube is subjected to repeated deformations, the deformed portion of the tube would deteriorate early. Also in the bellows type pump, its degradation cannot be avoided at the bellows portion which is subjected to larger deformation. Even if the bellows is made of stainless steel, a satisfactory durability would not be expected.